In a length-adjustable gas spring of this type, known from U.S. Pat. No. 3,656,593, and 3,711,054, the valve push rod is provided with an annular groove, located approximately in the center of the sealing plug, and serving as a bypass channel, by means of which annular groove a seal disposed facing the inner chamber of the housing and located in the sealing plug is bridged when the valve push rod is in the pushed-in position. A plate or thickening is provided at the inner end of the valve push rod, which, when the valve push rod is in the push-out position, in which the valve is closed, rests as a stop against the sealing plug, thus preventing the valve push rod from being pushed completely out of the gas spring. This known gas spring has proven extraordinarily reliable in practice; however, it has the disadvantage that the cost of manufacture is relatively high.